1. Field of the Invention
This invention relates generally to a system and method for detecting small anode leaks in a fuel cell system and, more particularly, to a system and method for detecting small anode leaks in a fuel cell system that looks at the rate of anode pressure decay at system shut-down.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode electrodes, or catalyst layers, typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, the gas diffusion layer (GDL), that protects the mechanical integrity of the membrane and also helps in uniform reactant humidity diffusion. MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. The fuel cell stack also receives an anode hydrogen input reactant gas that flows into the anode side of the stack. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a by-product of the chemical reaction taking place in the stack.
A fuel cell stack typically includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include anode side and cathode side flow distributors, or flow fields, for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is typically difficult to detect small hydrogen leaks in the anode of fuel cell systems. Large hydrogen leaks, such as a valve being open, can typically be detected using an anode inlet injector diagnostic or a flow meter. However, small anode leaks that might not be significant enough to exceed hydrogen emissions requirements, may not be detectable using an anode inlet injector diagnostic. Although small hydrogen leaks may not necessarily present an emissions problem, it is still desirable to correctly account for all hydrogen in the system for efficiency reasons and otherwise. Sources of small anode leaks could include small leaks through improperly seated valves, hydrogen cross-over through the membrane to the cathode side of the stack and/or small anode overboard leaks. Of these, hydrogen cross-over to the cathode is the most probable. Fortunately, cross-over leaks typically develop over tens to hundreds of operating hours, rather than seconds to minutes.
At fuel cell system start-up, assuming enough time has elapsed since the previous shut-down, most of the hydrogen remaining in the stack at the last shut-down has diffused out of the stack and both the cathode and anode flow channels are generally filled with air. When hydrogen is introduced into the anode flow channels at system start-up, the hydrogen pushes the air out of the anode flow channels creating a hydrogen/air front that travels through the anode flow channels. As described in the literature, the presence of the hydrogen/air front on the anode side combined with air on the cathode side causes electrochemical reactions to occur that result in the consumption of the carbon support on the cathode side of the MEA, thereby reducing the life of the MEAs in the fuel cell stack.
One known technique for significantly reducing the air/hydrogen front at system start-up, and thus, reducing catalytic corrosion is to reduce the frequency of start-ups in which the anode and cathode are filled with air. One strategy to achieve this is to leave the anode and cathode in a nitrogen/hydrogen environment. However, the hydrogen will eventually either diffuse out of the anode, or be consumed by oxygen slowly returning to the stack. Thus, in order to extend the ability to reduce catalytic corrosion, small amounts of hydrogen can be periodically injected into the stack while the system is shut-down. Because mostly nitrogen is remaining in the cathode side at system shut-down, as the result of the oxygen being consumed by the fuel cell reaction, nitrogen and hydrogen are the main elements that are equalized in the cathode and anode sides of the fuel cell stack after system shut-down. This does not allow oxygen in the air to form the air/hydrogen front.
When the fuel cell system is shut down, the gas permeation continues through the membrane until the gas component partial pressures have equalized on both sides of the membrane. The diffusivity of hydrogen through the membrane from the anode to the cathode is approximately three times the rate of nitrogen from the cathode to the anode. Higher hydrogen diffusivity rates equate to a rapid equalization of hydrogen partial pressures compared to a relatively slow equalization of nitrogen partial pressure. The difference in gas diffusivities causes the anode sub-system absolute pressure to drop until the cathode hydrogen partial pressure reaches the anode hydrogen partial pressure. Typically, the anode side of the fuel cell stack is operated at a high hydrogen concentration, such as greater than 60%, and large volumes of hydrogen-rich gas exist in the anode headers and anode plumbing outside of the anode of the stack. As the anode absolute pressure drops, more hydrogen is drawn out of the anode sub-system into the anode flow field of the stack.